Scanning probe microscopes (SPMs) detect an interaction (such as a tunneling current or an interaction force) acting between a sharp tip (probe) and a sample by moving the probe close to the sample, and feedback-control a distance between the probe and the sample such that the interaction is kept constant (in the following, the distance between the probe and the sample is referred to as probe-sample distance). SPMs also horizontally scan the probe (or the sample) while maintaining the feedback control. The probe (or the sample) thereby moves up and down so as to follow the contours of the sample. SPMs record a trajectory obtained by the feedback scanning with respect to a horizontal position, so that a contour image of the sample surface can be obtained.
A scanning tunneling microscope (STM) is one of SPM techniques. As shown in FIG. 1, in the STM, the interaction between the probe and the sample is a tunneling current. The STM detects a tunneling current flowing between the probe and the sample by applying a bias voltage between the probe and the sample, and controls the vertical position of the probe such that the tunneling current is kept constant.
FIG. 2 shows the relation between the probe position and the tunneling current. As shown in the drawing, the tunneling current flowing between the probe and the sample monotonically increases in an exponential manner with a decrease in the probe-sample distance. Accordingly, by controlling the vertical position of the probe relative to the sample such that the tunneling current is kept constant, the probe-sample distance can be kept constant.
Next, an atomic force microscope (AFM) will be described. The AFM is also one type of SPMs. The AFM detects an amount of interaction generated by an interaction force acting between the probe and the sample. Examples of the amount of interaction include a displacement amount of a cantilever, an amount of oscillation amplitude variation, an amount of phase variation, an amount of oscillation frequency variation or the like. The AFM feedback-controls the vertical position of the probe relative to the sample such that the detected amount of interaction is kept constant. The AFM employs a cantilever having a sharp tip (probe) at its end as a force detector.
FIG. 3 shows the relation between the probe position and the interaction force in the AFM. The relation is called force curve. FIG. 3 shows a typical example of the force curve measured in atmosphere and in vacuum.
As shown in FIG. 3, in atmosphere and in vacuum, when the probe is moved close to the sample, an attractive interaction force is normally applied first due to a van der Waals force and an electrostatic force. When the probe is moved closer to the sample, a strong repulsive force is applied due to a chemical interaction force, and the repulsive force exceeds the attractive force. Thus, the interaction force does not monotonically vary with the probe-sample distance.
When the STM and the AFM are compared, the tunneling current monotonically varies with the probe-sample distance in the STM as shown in FIG. 2. Meanwhile, in the AFM, the interaction force does not monotonically vary with the probe-sample distance as shown in FIG. 3. Thus, the amount of interaction generated by the interaction force does not also monotonically vary with the probe-sample distance, and it is difficult to stably feedback-control the probe position within an entire distance range. For example, in a distance region where the attractive interaction is dominant, the probe-sample distance is controlled on the assumption that the attractive force becomes stronger as the probe approaches the sample. The probe may be moved so much close to the sample under such control, as to enter a region where the repulsive force is dominant. In this case, the interaction force indicates an opposite response to a distance variation. The feedback control thereby becomes unstable, and the probe strongly collides with the sample.
Thus, in normal AFM observation, a feedback target point is selected from a distance region where the amount of interaction monotonically varies with the probe-sample distance for the sake of stable control. This means that only position information of a plane with an equal interaction amount where the amount of interaction has a given value is obtained, and information of a position where the amount of interaction has another value cannot be obtained. That is, information of positions other than the feedback target point cannot be obtained. For example, information of the lowest point and its proximity of the force curve in FIG. 3 cannot be obtained. Accordingly, in the conventional general AFM observation, information of the interaction force in a three-dimensional space cannot be obtained.
FIG. 3 shows the example in atmosphere and in vacuum. However, the above problem becomes more serious in AFM observation in liquid. In a solid-liquid interface, solvent molecules often form a layered structure. The interface thereby spreads not only horizontally but also vertically to the sample. In the conventional AFM technique, however, only the position information of the plane with an equal interaction amount where the amount of interaction has a given value can be obtained. Thus, the structure and physicality of the interface (more specifically, an interface space including the interface and its proximity) cannot be fully understood.
In the observation in liquid, the probe position is also sometimes not easy to control. FIG. 4 shows an example of a force curve in an interface where a layered structure of a hydrated layer or the like is formed. FIG. 4 shows the dependence of the interaction force on the probe-sample distance as a measurement result in phosphate buffered saline. As shown in FIG. 4, a repulsive force is generated when the probe penetrates the hydrated layer. After the probe penetrates the hydrated layer, an attractive force is generated. Due to the repulsive force and the attractive force, the force curve indicates a vibrating profile. Therefore, there are a plurality of probe positions usable in the feedback with respect to one interaction force. To stably perform the feedback control, one of the plurality of probe positions need to be selected with good controllability. However, such control is not easy to perform, and the controllability of the probe position is substantially lowered.
As described above, the normal AFM observation technique has a problem with the probe position control, and a problem that information of an interface that spreads three-dimensionally cannot be fully understood. To solve the problems, a three-dimensional space measurement technique based on force curve measurement has been conventionally proposed as described below.
By referring to FIG. 5, the conventionally-proposed three-dimensional measurement technique measures a distribution of interaction forces in a three-dimensional space by acquiring force curves at a plurality of measurement points arranged in an array on an XY plane. For example, as shown on the left side of FIG. 5, the force curves are measured while the X position is moved little by little with respect to the same Y position. By the operation, an XY image that reflects the distribution of interaction forces of an XZ plane can be provided. The similar operation is also performed while the Y position is shifted little by little. Accordingly, the XYZ three-dimensional distribution of interaction forces can be measured.
However, the conventional technique described above has a problem that it takes much time since the two operations (the operation to acquire the force curve at each point, and the operation to move the XY positions of the probe) need to be intermittently combined together.
There is also a problem that a great impulsive force is generated at a moment in which the probe is moved closest to the sample and at a moment in which the probe is moved farthest from the sample, to thereby induce oscillation in various machine parts.
To acquire the force curve, the probe is moved closer to the sample by a given distance from a given Z position regardless of the surface contours. Depending on the contours of the probe or the angle of the sample, the probe may strongly collide with the sample to cause great damage thereto.
To avoid the collision as described above, the probe may be moved away from the sample at a moment in which the interaction force exceeds a given value. However, to incorporate such control as to move the probe away from the sample, the interaction force needs to be monitored continuously during movement of the probe. Such monitoring control during measurement is more complicated than the normal simple force curve measurement where data is processed after the completion of measurement. Measurement time thereby increases, and as a result, a sample drift during measurement has a more influence. The above control to move away the probe also causes a problem that a great impulsive force is generated at a moment in which the probe moving direction is rapidly changed.
To avoid the problems as described above, the conventional three-dimensional measurement technique in FIG. 5 is applied within a correspondingly limited range. Actually, the conventional three-dimensional measurement technique has been employed only in AFM observation under an extremely-low temperature environment in ultrahigh vacuum where the influence of drift can be ignored even when it takes very long time to measure the force curve. Accordingly, it is very difficult to use the conventional three-dimensional measurement technique in atmosphere and in-liquid environments. It is also very difficult to use the technique in a room-temperature environment.
The aforementioned conventional three-dimensional measurement technique using the force curve is disclosed in Non Patent Literature 1, for example.
The background art of the present invention has been described above based on the AFM. However, similar demands may also arise in other types of SPMs.